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* CHEMISTRY. 575 CHEMISTRY. CHEMISTRY, Ackk ri/riRAL. The branch of chemical Miiiuc which deals with the prohleins of the soil, the nutrition of plants and animals, the composition of their (iroducts, and their value as food for man and animals. Its field borders on those of vef;etiihle and animal physiol- ogj- on the one hand, and of physics and <;eolooy on the other: and in workinf; out pioblems in a}»ricult>ire. the aaricnltural chemist has fre- quently b<M'n led into these adjoininj; branches of science. His work has, however, been essentially chemical, and thus a quite distinct and well- defined branch of chemical science has been built up. Hi.sTonv. The first attempt to bring together in a systematic manner the teachings of chem- istry as applied to agriculture was made by Sir Humphry Davy, in a series of lectures before the British Board of Agriculture, early in the Nineteenth Century {Kltmciits of AgricuUural Chemistry, 1814). Bavy regarded the soil as the source of all nourishment of plants, and he announced the general principle that plant food must be practically dissolved Ijcfore it can enter the organism of the plant. But while he and his predecessors knew the value of certain ma- terials for plant food, they did not know the constituents which gave them this value. Oil, for e.vaniple, was long supposed to be one of the most valuable fertilizing constituents, and Davy supposed sugar to be also. One of the reasons for this error was undoubtedly the belief that the carbon of plants was derived through the roots from the humus — i.e. the partially decom- posed organic matter of the soil. The errors of this 'humus theory' were pointed out, about 1840, by .Justus von Liebig, who in his generalizations drew heavily upon the works of Saussure and Boussingault. Liebig showed the carbonic acid of the air to be the source frohi which plants derive their carbon, and the leaves to be the medium through which this assimilation takes jilaee. He thus laid down a fundamental prin- ciple of physiological botany. He established the interdependence of plants and animals with relation to carbonic acid, which is a vitalizing principle of the one and an excretory pioduct of the other. In place of the iiumus theory' he advanced his famous 'mineral theory,' accord- ing to which the soil furnishes to plants their ash or mineral constituents: and this led him to suggest the employment of artificial fertiliz- ers for keeping uj) the supply of the mineral con- stituents of the soil, and thus maintaining its fertility. Liebig's views, with some modifica- tions, form the basis of our present theories; but. on the other hand, we have learned much re- garding the true value of hunuis and its func- tions in the soil. See M.^xtuES. In the field of animal nutrition, or the utili- zation of plants and their products in feeding animals, agricultural chemistry has likewise led to the establishment of the underlying principles and the methods of research. The earlier ideas regarding the valuable food constituents of plants were equally as cnide as those regarding the fer- tilizing constituents. There was a prevalent be- lief in a subtle, undefined property of forage plants, upon which their nutritive properties largely depended, and it was a hmg time before the connection between nutritive value and chemical composition was traced. Boussingault, in France, attempted to classify feeding stuffs on the basis of their nitrogenous constituents; and Thaer, in Germany, compared them with hay, and prepared his tables of "hay values.' At length Henneberg worked out a classification of the nutritive constituents of feeding stulfs, and a method of analysis known as the W'eende method. His classification and method have been extensively emjiloycd, having .sened as the basis in most of the investigations since. These studies in animal nutrition have led the agricultural chemist into a field of inquiry intimately con-, nectcd with physiology' proper; and in the de- velopment of the present theories, as well as in their application in practice, agricultural chem- istry has coiitriliulcil nuich more than physiology it-self, and has usually been the leader. Agricultural ehemistiy laid the fcunidation for a science of agriculture, and one of the most im- portant and far-reaching influences of the early agricultural chemists was that which led to the establishment of the agricultural experiment station. The teaching of agriculture in agri- cultural schools and colleges preceded it in most countries, but the investigation and experimen- tation was for the most part in the hands of the agricultural chemists. Their work led them into agricultural experiments of various kinds, as a matter of necessity, and Liebig, Boussin- gault, Lawes and Gilbert, and others, laid out experimental fields for carrying on their studies and testing their theories. As the practice of fertilizing the soil became introduced in the Old World, farmers found themselves confronted by a variety of problems relating to their soils and fertilizing materials which they were unable to answer, and they quite naturally turned for ad- vice and guidance to the agricultural chemists Avho had developed these theories. Thus exjieri- ment stations came to be established, first by agricultural societies of private individuals, and later with Government aid. (See AoRicuLTrR.i^L ExpERiMEXT Statioxs.) In the United Stiites, the researches and writings of S. W. Johnson I Hotc Crops Grow, 1869: How Crops Feed, 1870), C. A. Goessmann, E. V. Hilgard. G. C. Caldwell, W. O. Atwater, and other chemists, ])repared the way for the establishment of ex- of these has grown a system of stations under Government aid and supervision, embracing eveiy State and Territory in the L'nion, and providing, incidentally, the most numificent endowment for agricultural-chemical research to be found in any country. AcRiCTJi.TURAi, Analysis. The chemical sub- stances with which agricultural ehemistiy is primarily concerned are those comprising the ash, or mineral constituents, and the food con- stituents of i)lants. As the soil is the source of the mineral constituents of plants, and usually of the nitrogen also, those elements of the soil which serve as food for plants — i.e. which enter into their composition — are embraced in the field of agricultural chemistry: and. on the other hand, as farm animals derive their nourishment from lilants. ai.'ricultural chemistry is naturally con- cerned with the fiKid constituents of these plants. The plant is thus a link between the soil and the animal, as regards the mineral elements: but the j)lant gets its carbon from the air. and develops from it a series of carbon-containing compounds, which, in turn, arc the source of carbon to the animal.
 * )eriment stations as State institutions; and out